Abstract [en]

We present observations by the Fermi Gamma-Ray Space Telescope Gamma-Ray Burst Monitor (GBM) of the nearby (z = 0.55) GRB 101219B. This burst is a long GRB, with an associated supernova and with a blackbody (BB) component detected in the early afterglow observed by the Swift X-ray Telescope (XRT). Here we show that the prompt gamma-ray emission has a BB spectrum, making this the second such burst observed by Fermi GBM. The properties of the BB, together with the redshift and our estimate of the radiative efficiency makes it possible to calculate the absolute values of the properties of the outflow. We obtain an initial Lorentz factor G = 138 +/- 8, a photospheric radius r(phot) = 4.4 +/- 1.9 x 10(11) cm, and a launch radius r(0) = 2.7 +/- 1.6 x 10(7) cm. The latter value is close to the black hole and suggests that the jet has a relatively unobstructed path through the star. There is no smooth connection between the BB components seen by GBM and XRT, ruling out the scenario that the late emission is due to high-latitude effects. In the interpretation that the XRT BB is prompt emission due to late central engine activity, the jet either has to be very wide or have a clumpy structure where the emission originates from a small patch. Other explanations for this component, such as emission from a cocoon surrounding the jet, are also possible.